This application is based on Application No. 2001-001638, filed in Japan on Jan. 9, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an air/fuel ratio control apparatus for an internal combustion engine which can detect the air/fuel ratio of a mixture in each cylinder based on the output of an air/fuel ratio sensor installed on a collective portion of an exhaust system at which exhaust gases from a plurality of cylinders are merged with each other, and uniformly control the air/fuel ratios of all the cylinders to a target value, and in particular, it concerns with such an air/fuel ratio control apparatus for an internal combustion engine which is improved in the reliability in control by the use of relatively easy determination processing.
2. Description of the Related Art
In general, with an air/fuel ratio control apparatus for an internal combustion engine (hereinafter simply referred to as an engine), there has been known a method for controlling an air/fuel ratio of an air fuel mixture supplied to each cylinder of the engine in the following manner. That is, an air/fuel ratio sensor is installed on a collective portion of an exhaust system of the engine, and the control apparatus operates to take in an output (i.e., air/fuel ratio signal) of the air/fuel ratio sensor in synchronization with the timing at which exhaust gases from respective cylinders of the engine pass through the exhaust system collective portion, and correct the amounts of fuel to be injected into the respective cylinders in response to the air/fuel ratio signal from the air/fuel ratio sensor thereby to uniformly control the air/fuel ratios of the air fuel mixture supplied to the respective cylinders.
At this time, it is required to accurately detect the air/fuel ratio of an exhaust gas corresponding to each cylinder, but it is known that in actuality, the exhaust gases from a plurality of cylinders in a state of being mixed with one another pass through the exhaust system collective portion (i.e., air/fuel ratio sensor), so the air/fuel ratio detection information obtained from the air/fuel ratio sensor includes the influence of the air/fuel ratios in the respective cylinders, and hence it is difficult to accurately detect the air/fuel ratio in each cylinder.
In addition, since the extent of the influences of other cylinders on the detected value of the air/fuel ratio of a specific cylinder varies according to the engine operating state (e.g., the number of revolutions per unit time of the engine, the amount of intake air, etc.), it is even more difficult to accurately detect the air/fuel ratio corresponding to the specific cylinder.
Moreover, since the air/fuel ratio sensor has a time lag (i.e., so-called response delay) from the time when an exhaust gas has actually arrived there to the time when the air/fuel ratio of the exhaust gas detected by the sensor appears as a detection value, it is necessary to take such a response delay into consideration and thus it becomes very difficult to detect the air/fuel ratio corresponding to each cylinder in a reliable manner.
Therefore, it is difficult for the conventional apparatus to accurately detect the air/fuel ratio corresponding to a specific cylinder, thus making it impossible to achieve a high degree of air/fuel ratio control accuracy to any satisfactory extent.
To cope with such a problem, apparatuses were proposed in the past in Japanese patent Nos. 2689362 and 2717744, for example.
In the apparatuses described in the above-mentioned patents, there is constructed a mathematical model representing the behaviors of component elements such as the flow of exhaust gases to an exhaust system, an output response delay of an air/fuel ratio sensor, etc., based on which an observer is designed to estimate an air/fuel ratio in each cylinder, whereby the air/fuel ratios in the respective cylinders are made uniform.
In the case of using such a mathematical model, however, the arithmetic operations in a central processing unit (CPU) become complicated, resulting in an increased processing time and an increased software load. Moreover, since the behaviors of respective component elements of the apparatus are expressed by the mathematical expressions, the results of estimation carried out by the use of the mathematical model are liable to various influences such as a change in the operating state of the engine, variations in the manufacture of component elements, etc., and hence it is impossible to obtain a satisfactorily high degree of control accuracy.
On the other hand, it was also proposed that as disclosed in Japanese Patent Publication No. 4-8616 for instance, the output value of an air/fuel ratio sensor and the air/fuel ratios of respective cylinders can be correlated with each other easily by limiting the requirements for detecting the air/fuel ratios to an idling state of the engine alone, thereby achieving uniform control on the air/fuel ratios in the respective cylinders.
However, in the idling state of the engine, the combustion state is unstable so the engine rotation is liable to undergo great fluctuations, as a consequence of which there are a lot of factors deteriorating the accuracy in the detection of the air/fuel ratios, and there is a decreased amount of exhaust gases in that engine operation range, thus further worsening the response of the air/fuel ratio sensor.
Therefore, even if the air/fuel ratio is detected in the idling state, the width or magnitude of variations in the sensor output will become small and hence it is difficult to accurately detect the information on an air/fuel ratio corresponding to each cylinder after all.
FIG. 14 is an explanatory view illustrating a correction target cylinder determining operation performed by a conventional apparatus which adopts a method of detecting air/fuel ratios at respective prescribed crank angles, in which the abscissa represents reference crank angle positions (TDC) corresponding to the respective cylinders (#1 through #4), and the ordinate represents a change in the output value of an air/fuel ratio sensor (air/fuel ratio A/F).
In FIG. 14, arrows of broken lines indicate the air/fuel ratio detection timing of the respective cylinders, and the output value (A/F) of the air/fuel ratio sensor is read at the respective prescribed crank angles.
Herein is shown the case where the air/fuel ratio in the cylinder #3 is shifted or deviated to a rich air/fuel ratio side (hereinafter simply referred to as a rich side) as compared with those in the remaining cylinders.
According to such a method of detecting at the respective prescribed crank angles, there might be a fear that though the cylinder #3 is to be determined as a cylinder corresponding to a rich side peak phase, the cylinder #4 is mistakenly determined as a correction target cylinder, for example, in the case where the air/fuel ratio of the cylinder #4 is shifted or deviated to the rich side more than that of the cylinder #3, as indicated by solid line arrows.
In the conventional air/fuel ratio control apparatus for an internal combustion engine as described above, in the case where a variety of behavior factors are taken into a mathematical model, the load on software increases, and the apparatus is subject to influences such as a change in the engine operating state, variations in the manufacture of component parts, etc., which are difficult to represent by means of mathematical expressions, thus making it impossible to achieve sufficient reliability.
Moreover, when the air/fuel ratio detection condition is limited to an idling state of an engine, there arise the following problems: namely, in the idling state in which the combustion state of the engine is unstable and the amount of exhaust gases is limited, the rotational speed of the engine varies greatly, thus deteriorating the detection accuracy, and the variable range of the air/fuel ratio sensor output is small, as a consequence of which it is difficult to accurately detect the information on the air/fuel ratios corresponding to the respective cylinders.
The present invention is intended to obviate the problems as referred to above, and has its object to provide an air/fuel ratio control apparatus for an internal combustion engine which is capable of making the air/fuel ratios of respective cylinders uniform with high accuracy without causing an increase in the load of arithmetic operations of a CPU, and without being subject to the influence of a change in the engine operating state, etc., by specifying as a fuel amount correction target a cylinder for which the air/fuel ratio deviates from those of the remaining cylinders, based on a variation in the output value of the air/fuel ratio sensor.
Bearing the above object in mind, according to the present invention, there is provided an air/fuel ratio control apparatus for an internal combustion engine, comprising: fuel injectors respectively provided on a plurality of cylinders; an air/fuel ratio sensor disposed at a collective portion of an exhaust system at which exhaust gases from the respective cylinders are collected; a crank angle sensor for detecting a crank angle phase of each of the cylinders; an air/fuel ratio detection period setting section for setting an air/fuel ratio detection period in which exhaust strokes of all cylinders are included; a peak phase detecting section for taking in an output of the air/fuel ratio sensor during the air/fuel ratio detection period and detecting peak phases which become a maximum on a rich side or on a lean side due to variations in the air/fuel ratio; an air/fuel ratio correction cylinder determining section for specifying, based on the peak phases, an air/fuel ratio correction target cylinder for which an air/fuel ratio is to be corrected; and a cylinder-specific air/fuel ratio correcting section for correcting an amount of fuel to be supplied to the air/fuel ratio correction target cylinder. The amounts of fuel to be supplied to the respective cylinders are made uniform by means of the cylinder-specific air/fuel ratio correcting section.
In a preferred form of the present invention, the cylinder-specific air/fuel ratio correcting section corrects an amount of fuel to be supplied to the air/fuel ratio correction target cylinder in a decreasing direction when the air/fuel ratio correction target cylinder corresponds to a rich side peak phase, but in an increasing direction when the air/fuel ratio correction target cylinder corresponds to a lean side peak phase.
In another preferred form of the present invention, the peak phase detecting section detects a rich side peak phase alone, and the air/fuel ratio correction cylinder determining section specifies, based on the rich side peak phase, only that cylinder for which an output value of the air/fuel ratio sensor is assumed to deviate from those for the remaining cylinders, as the air/fuel ratio correction target cylinder.
In a further preferred form of the present invention, the peak phase detecting section detects a lean side peak phase alone, and the air/fuel ratio correction cylinder determining section specifies, based on the lean side peak phase, only that cylinder for which an output value of the air/fuel ratio sensor is assumed to deviate from those for the remaining cylinders, as the air/fuel ratio correction target cylinder.
In a yet further preferred form of the present invention, the peak phase detecting section detects a rich side peak phase and a lean side peak phase, and when it is determined that a first cylinder and a second cylinder correspond to the rich side peak phase and the lean side peak phase, respectively, and when the order of respective exhaust strokes of the first and second cylinders is consecutive, the air/fuel ratio correction cylinder determining section specifies only one of the first and second cylinders, of which the exhaust stroke is later than that of the other cylinder, as the air/fuel ratio correction target cylinder.
In a still further preferred form of the present invention, when it is determined that a first cylinder and a second cylinder correspond to the rich side peak phase and the lean side peak phase, respectively, and when the order of respective exhaust strokes of the first and second cylinders is not consecutive, the air/fuel ratio correction cylinder determining section specifies the first and second cylinders as the air/fuel ratio correction target cylinders, for which the air/fuel ratios deviate to the rich side and the lean side, respectively.
In a further preferred form of the present invention, when it is determined that a first group of cylinders and a second group of cylinders correspond to the rich side peak phase and the lean side peak phase, respectively, and when the first group of cylinders including one half of a total number of all the cylinders and the second group of cylinders including the other half of all the cylinders are detected alternately, the air/fuel ratio correction cylinder determining section specifies either one of the first and second groups of cylinders as the air/fuel ratio correction target cylinders.
In a further preferred form of the present invention, the peak phase detecting section detects a rich side peak phase and a lean side peak phase, and the cylinder-specific air/fuel ratio correcting section calculates deviations of the respective output values of the air/fuel sensor corresponding to the rich side peak phase and the lean side peak phase, and stops the fuel amount correction processing when the deviation becomes equal to or less than a predetermined value.
In a further preferred form of the present invention, the air/fuel ratio detection period setting section comprises a window setting section for setting a window corresponding to each of the cylinders for detection of the peak phases, the window being set variably according to an engine operating state.
In a further preferred form of the present invention, the window setting section sets a first window corresponding to one of the cylinders, and then sets windows corresponding to the remaining cylinders by successively adding to the first window a crank angle difference between the one cylinder and each of the remaining cylinders.
With the above-mentioned respective arrangements, there is obtained an air/fuel ratio control apparatus for an internal combustion engine which can make the air/fuel ratios of the respective cylinders uniform with high accuracy without causing an increased operation load of a CPU, while avoiding the influence of a change in the engine operating state, etc.
The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of preferred embodiments of the invention taken in conjunction with the accompanying drawings.